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Air-barrier rocket nozzle

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So, I have been toying with the possibility that nobody may win the N-Prize, leaving it up to me.

One of the problems with rocket engines, particularly ones fuelled by LOX and a liquid fuel, is that the combustion chamber and rocket nozzle usually have to be cooled to prevent them burning through.

Traditionally, this is done by circulating either the fuel or the LOX through pipes on the outer surface of the chamber and nozzle, on their way to the combustion chamber. This is quite neat (it cools the critical parts, and pre-heats the fuel or oxygen), but also complex. It's particularly problematic for small engines.

So, how to avoid this?

The desirable option would be to line the combustion chamber and nozzle with Starlite, a heatproofing compound of truly remarkable fortitude (linky). However, the inventor is dead, and during his lifetime he never revealed the secret of its composition, except to his family.

So, Plan B. What happens if you take a conventional rocket combustion chamber, and cut a slot in it, circumferentially, near the top, to let air in?

Obviously, when the rocket is sitting on the launch pad firing away, hot exhaust gasses will come out of this slot, and it will all be a bit of a waste. However, once the rocket is travelling at speed, air should be forced into the slot instead. This air would form a boundary layer around the hot gases, effectively working as a barrier between the gases and the walls of the chamber and nozzle.

There are obviously some issues here, because the combustion chamber is, necessarily, at very high pressure. Hence, an air scoop might be needed to pressurize the oncoming air and force it through the slot.

Ultimately, this design sort of approaches a ramjet, except that the oxidiser is LOX rather than the inrushing air itself. It also sort of nods toward an aerospike design, where the surrounding air performs the function of a containing "nozzle".

MaxwellBuchanan, Sep 05 2012

Starlite http://www.youtube....watch?v=W4nnLP--uTI
A few mm keeps an egg uncooked after 3min in a blowtorch flame. [MaxwellBuchanan, Sep 05 2012]

Air-augmented rocket https://en.wikipedi...ir-augmented_rocket
What this is a reinvention of [notexactly, Jan 13 2016]

[link]






       Can you get enough air at the altitude the engine will be travelling through to act as a cooling agent? Would richening the mixture work like it does on piston engines? There is a flux for welding thin stainless exhaust tubing for racing engines that is applied to the back of the joint and becomes glass-like during the weld preventing burn through and weld drooping on the inside of the joint. The product in the video needs to be located. From the family?
cudgel, Sep 05 2012
  

       //Can you get enough air...?//   

       Good question. However, the rocket is accelerating, which would scoop in more air, perhaps compensating for the lower atmospheric density.   

       //Would richening the mixture work like it does on piston engines? // I don't know. But this would seem to rely on incomplete combustion to reduce temperatures inside the chamber and nozzle, which would lower efficiency.
MaxwellBuchanan, Sep 05 2012
  

       //However, the inventor is dead, and during his lifetime he never revealed the secret of its composition, except to his family.//   

       If samples have been given to NASA then its composition should be known.
Aerogel powder maybe?
  

       //which would lower efficiency// Agreed, this is a crutch sometimes used on piston engines. Has burn time been calculated to be long enough for available nozzle material to fail?
cudgel, Sep 05 2012
  

       True rockets are used for orbital lift because their dwell time in the atmosphere is short, and once exoatmospheric there is no air…   

       Yes, it's a ramjet.   

       On the pad, the counterthrust back up your "scoop duct" will add drag. The scoop will add drag … and won't work below supersonic speeds. Plus it will add weight and complicate the vehicle design.
8th of 7, Sep 05 2012
  

       I bored NASA with something similar in 2003, a sleeve that extended down from the nozzle, channel air into it via NACA ducts, try and get some ramjet effect off the hot sleeve.   

       They have since added an extra-large bin marked "Crank" to deal with my kind...   

       //Starlite   

       Maybe it's just a heat superconductor, to this realm, or another.
not_morrison_rm, Sep 06 2012
  

       // This is quite neat (it cools the critical parts, and pre-heats the fuel or oxygen), but also complex. //
It's just a big Primus stove, shirley?
AbsintheWithoutLeave, Sep 06 2012
  

       Is it really likely that no one will win the N-Prize?
hippo, Sep 06 2012
  

       //If samples have been given to NASA then its composition should be known.// The inventor did not let samples out of his sight, apparently, and did not give anyone the chance to reverse engineer the stuff.   

       //Has burn time been calculated to be long enough for available nozzle material to fail?// Apparently yes. All significant liquid rockets seem to have cooled chambers and nozzles, and there are many articles on trying to to find materials which can be operated uncooled, so it's clearly still an issue.   

       //The scoop will add drag … and won't work below supersonic speeds// Yes, I was afraid of that. Ah well.   

       //I bored NASA with something similar in 2003// Great minds think alike.   

       //Is it really likely that no one will win the N- Prize?// Hard to tell. There's one more year to run (after extending the prize for a final further year), and a handful of teams look like serious contenders. But if not, I shall have to buy earplugs and hard hats for everyone on the Estate, and get down to some serious foolhardiness.
MaxwellBuchanan, Sep 06 2012
  

       //I bored NASA with something similar in 2003// Great minds think alike.   

       If that includes me, you can expect a visit from the Trading Standards people very soon.
not_morrison_rm, Sep 06 2012
  

       OK, so this isn't going to fly.   

       How about plan C. Hydrogen peroxide is used as a monopropellant, and decomposes at a fairly low temperature. But it's not as good as LOX+hydrocarbon.   

       However, suppose you took a regular LOX/fuel rocket, and added a ring of peroxide injectors around the edge? Then the decomposing peroxide would act as a barrier to protect the casing from the LOX/fuel combustion, but the peroxide would be adding a fair amount of additional thrust, rather than being dead weight or drag.
MaxwellBuchanan, Sep 06 2012
  

       // act as a barrier //   

       Er, how exactly?   

       Let's say that by some perverse quirk of laminar flow, you can create and maintian a 10mm thick layer of cool, liquid H2O2 on the inner surface of the bell.   

       But the main combustion is going to be going on very close, and throwing out quite staggering amounts of infrared, to which the peroxide isn't completely opaque. So although you may reduce input by conduction, the radiation transfer is still going to be significant.   

       If you could maintain an Argon jacket in the interior at the same pressure as the exhaust, that might do some good. The Argon is pretty non-conductive, and would keep free radicals away from the walls too.
8th of 7, Sep 06 2012
  

       //If you could maintain an Argon jacket in the interior// Is that like a Nylon jacket? Cuz if it is I could donate a couple to the project.   

       Instead of creating a barrier or sacrificial lining to the nozzles, why not try a design a nozzle which continually turns inside out for cooling whilst still remaining the desired shape? Something like a Caterpillar tread or perhaps a series of roller bearings.
AusCan531, Sep 06 2012
  

       Argon is pretty well the worst thing to use for coo... hmm... you mean an Argon _layer_, which goes out with the exhaust. Okay, I'm in, but your speed of sound just went down the tubes if you've an H2+O rocket.
FlyingToaster, Sep 06 2012
  
      
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